Many industrial processes produce waste air streams, often hot, containing low concentrations of organic solvents. For example, solvent containing air streams are produced as a result of solvent vaporization in the drying of synthetic fibers and films, plastics, printing inks, paint lacquers, enamels and other organic coatings. In addition to being a pollution problem, these streams represent a waste of valuable resources in the form of lost solvent and in the wasted energy in the heated air. The total value of the solvent and heat loss in these processes is very large. For example, it has been estimated that 200 million barrels per year of solvent are being recovered by the existing processes and that an even larger volume of solvent is being discharged or lost.
Since the early 1970's the industries producing solvent containing exhaust air streams have been under increasing economic and regulatory pressure. One problem is the increasing cost of energy. Many of these streams are produced in high-temperature industrial ovens where, because of the explosion danger, strict limits govern the concentration of solvent vapors permitted in the oven. One method used to stay within these limits is to ventilate the oven chamber with fresh air in sufficient volume to dilute the maximum vapor concentration to acceptable levels. This method wastes large amounts of process heat in the exhaust gas. Of even greater economic significance is the solvent contained in these exhaust streams. In the past, these organic solvent vapors were simply discharged because air pollution regulations were lax and the solvents were inexpensive. Presently, however, some form of treatment is required to meet air pollution standards. Most of these processes only prevent air pollution, and despite the increased value of the solvent, its recovery is still not economically practical except for very large exhaust streams and under particularly favorable conditions.
One could, in principle, recover the solvent from oven exhaust air directly by compressing the entire air stream to a pressure at which the solvent would condense. However, effluent streams typically contain only small volumes of organic solvent, such as 1 volume % organic solvent vapor, and because of the large amounts of energy required to highly compress such a large volume of gas, this approach is economically impractical.
The United States Environmental Protection Agency (EPA) has published a whole series of reports on the problem of solvent vapor emissions. One of the most pertinent is "Control of Volatile Organic Emissions from Existing Stationary Sources--Volume 1: Control Methods for Surface-Coating Operations", EPA-450/2-76-028, November 1976, which contains a description of the solvent vapor recovery systems known in 1976. The vapor control systems described are incineration, carbon adsorption, condensation, and scrubbing. Of these, incineration and carbon adsorption are the most widely used processes. In incineration , the vapor-containing stream is mixed with natural gas and burned in a high temperature incinerator. In carbon adsorption, the feed solvent vapor stream is passed through a bed of high surface area carbon beads onto which vapor is sorbed. Periodically, the carbon bed is eluted with steam or hot gas to produce a concentrated product containing the adsorbed solvent. Both processes are widely used, but are expensive.
The high permeability of some rubbery polymers, particularly silicon rubber and polyacrylonitrile (pan)butadiene, to organic vapors and their low permeability to nitrogen and oxygen is known. See, for example, Rogers et al, "Separation by Permeation Through Polymeric Membranes", in Recent Developments in Separation Science, Volume II, pages 107 to 155 (1972), and the paper by Spangler, American Lab 7, 36, 1975. Rogers et al, for example, disclose that the permeability of poly(butadiene-acrylonitrile, 35%) rubber to nitrogen and oxygen is much less than to certain organic materials such as methanol, carbon tetrachloride, ethyl acetate, benzene and methyl ethyl ketone. Spangler discloses trace vapor detectors employing dimethylsilicone membrane separators for detecting 2,4,6 TNT or DNT in ambient air. Despite this theoretical knowledge, however, no practical sytem for using such characteristics in a solvent recovery sytem is known.
Composite membranes are also known in the art. These membranes are usually used in reverse osmosis systems, but they have also been used for gas separation. Typical composite membranes are disclosed by Riley et al, "Permeability of Plastic Films and Coatings", in Polymer Science and Technology, Volume 6, page 375 to 388 (1974), U.S. Pat. No. 4,243,701 to Riley et al and Ward et al, J. Membr. Sci., Volume 1, pages 99 to 108, 1976. Composite membranes generally comprise a thin barrier layer of a permselective membrane and a microporous membrane support layer. The Riley et al article discloses a composite membrane comprising a porous cellulose nitrate-cellulose acetate supporting membrane and a thin semipermeable barrier of cellulose triacetate which can be formed directly in a thickness of about 250.degree. .ANG. to 500.degree. .ANG. upon the finely porous surface of the support membrane by dipping or by wicking from a dilute solution of cellulose triacetate in chloroform. The composite membrane can be given a spiral-wound construction and is employed in reverse osmosis for single-stage seawater desalination. The Riley at al patent discloses composite membranes which can be used for the separation of gases comprising a porous support membrane of cellulose nitrate-cellulose acetate or polysulfone and a thin film of a semipermeable material such as dimethyl silicone rubber. The patent indicates that the composite can be used in the form of a spiral wound element, and discloses selectivities of dimethyl silicone polymer which vary from 2.0 for O.sub.2 /N.sub.2 up to 50 for SO.sub.2 /N.sub.2. The Ward et al article discloses composite membranes in which the barrier layer comprises an ultrathin silicone-polycarbonate membrane, and suggests that the composite membrane can be used to produce oxygen-enriched air or nitrogen-enriched air.
Spiral wound modules are known in the art and have already been applied to the separation of gases, for example, by the Separex Corporation which in a brochure has described the use of a cellulose acetate membrane to separate hydrogen and CO.sub.2 from gases such as methane, ethane and CO.
U.S. Pat. No. 3,903,694 to Aine describes a method of recycling some of the unburnt hydrocarbons in the engine exhausts to the air inlet gas of the engine. Aine discloses that the process preferably is a concentration driven process rather than a pressure driven process. Thus, both the feed gas and the exhaust gas are close to ambient pressures. This means that only a portion of the hydrocarbon in the exhaust gas can diffuse across to the air inlet gas before both sides have the same hydrocarbon concentration and the process stops. For example, if the exhaust gas contains 1000 ppm hydrocarbon, then (assuming the exhaust gas and the feed gas volumes are approximately constant), the process will stop when the air inlet gas and the exhaust gas both reach 500 ppm hydrocarbon. This process is therefore a method of only recycling a portion of the hydrocarbon, at best 50%, and in practice probably a lot less. Moreover, this is not a process for concentrating the hydrocarbon vapor. The concentration of hydrocarbon on the air inlet (product side) of the membrane must always be less than on the exhaust (feed side) of the membrane. The Aine patent does disclose as a non-preferred embodiment, the possibility of employing a reduced pressure on the product side, but does not disclose recovery of the separated product as a liquid or a method of achieving high concentrations of organic vapor in the product.
U.S. Pat. No. 2,617,493 describes a process for removing nitrogen and other gases from hydrocarbon feed streams that generally contain 50% or more of the hydrocarbon gas. In this patent, because of the very high value of the organic feed, no hydrocarbon can be lost with the nitrogen. Thus, a multi-stage process is described to obtain a complete separation between the two components. This multi-stage system is economically impractical for feed streams containing low concentrations of components to be recovered. The membranes described in this patent are preferably between 12.5 and 123 .mu.m thick.
Barrier membranes have been reported in the literature that appear to have a high organic vapor to nitrogen selectivity, .alpha., defined as ##EQU1## where P.sub.vap and P.sub.N2 are measured separately on the pure vapor or nitrogen streams. However, this high apparent selectivity would not be expected to hold when the membranes are tested with vapor/nitrogen mixtures. This is because, with these mixtures, the high sorption of organic vapor by the membrane would be expected to swell the membrane so drastically that the membrane would no longer be a selective barrier to nitrogen.
Thus, despite the various diverse teachings in the prior art relating to the problem of solvent vapor emissions and the availability of membranes highly permeable to organic vapors, there has not been a process which recovers organic vapor at low concentrations from air by use of membrane technology.